In connection with heavy duty highway vehicles, such as large tractor-trailer rigs, excellent brake technology has developed on the basis of high-pressure air systems which power the brakes of both the tractor and the trailer. Such systems are, in general, too expensive to install on pleasure cars and light trucks which are occasionally used to tow vacation trailers or the like. It is of definite economic advantage to be able to use the auxiliary power facilities of an ordinary motor car, such as the 12 volt electrical system, engine intake manifold vacuum, and brake or power steering hydraulic systems to assist in the operation of trailer brakes.
The problems involved in employing these facilities to operate a trailer brake system to full satisfaction continue to trouble the related industries. The approach taken in developing the present invention is to provide the trailer with an independent braking system, preferably hydraulic, including a power booster from which hydraulic affect is transmitted to individual wheel brake mechanisms. Such transmission of braking effort may also be done by known rod or cable means connecting the booster with the wheel brakes. Present technology provides suitable electrical transducers (see U.S. patent application Ser. No. 699,897 filed June 25, 1976) and well developed electrical amplification equipment, but there is a lack of suitable apparatus for accomplishing the reverse transduction of electrical signals transduced from the operation of the tow vehicle's brake system to appropriate responses of the booster unit of the trailer brake system. If vacuum power from the tow vehicle is utilized as a medium in such reverse transduction, then the essential problem is in obtaining pneumatic pressures for operation of the booster unit which are proportional to the electrical signals produced in the tow vehicle. Also entailed is the necessity for sustaining pneumatic pressures at any level needed for maintaining desired braking effort.
It is an object of this invention to provide a braking system carried by a tow vehicle and trailer therefor for operation of the trailer brakes wherein the trailer brakes may be controlled solely by electrical and pneumatic, preferably vacuum, power supplied by the tow vehicle.
Another object is to provide a trailer brake system wherein the portion carried by the trailer comprises a booster unit and brake parts actuated thereby as a subsystem independent of the tow vehicle except for transducing signal media and a source of positive or negative fluid pressure.
It is also an object to provide a trailer brake system which may be actuated to braking effort proportional through a desired range to that imposed through a corresponding range of brake effort on the tow vehicle; also to be able to change the magnitude of trailer braking action in respect to any selected level of braking effort of the tow vehicle while retaining proportionality throughout a range of braking effort; and also to be able to operate the trailer brake separately from the tow vehicle brakes through manual adjustments of elements of the control apparatus.
Another object is to provide a trailer brake system that is extremely responsive and accurate in response to demands for braking effort throughout its operating range.
It is also an object to have a trailer brake system to be free of lag in operation (hysteresis) due to friction, etc. of control components, and to have very low hysteresis from any source at low braking effort.